(1) Field of the Invention
The present invention relates to an image forming apparatus which is applied to the printer portion of digital copiers and facsimile machines, digital printers, plotters and the like, and in particular relates to an image forming apparatus which creates images on the recording medium by making the developer jump thereto.
(2) Description of the Prior Art
Recently, as an image forming apparatus which reproduces an image signal into a visual image output on a recording medium such as paper etc., an image forming apparatus has been proposed in Japanese Patent Application Laid-Open Hei 5 No. 134,581. The image forming apparatus in this disclosure directly forms toner images on the recording medium by generating an electric field for toner as a developer so as to make the toner jump by electric force whilst a control electrode which has a plurality of passage holes and is arranged in the toner transfer path has varying potentials applied thereto.
The above conventional apparatus includes a developing sleeve, an opposing electrode and a control electrode having openings therebetween. In this conventional art, the voltage to be applied to an opening of the control electrode is switched between a first level of potential which allows toner to pass through the opening and a second level of potential which will not allow toner to pass therethrough so as to control toner transfer thus making it possible to directly form a toner image on the surface of paper.
In the above technology, use is made of the opposing electrode which is made up of a mesh-like roller and a decompressing means disposed inside the roller. In this arrangement, the recording medium is adapted to be conveyed by the rotation of the roller whilst being attracted to the roller by making use of suction force generated by air suction through mesh holes.
In image forming apparatus of the type which directly forms images by making toner jump as typically represented by the above conventional technology, the image is formed by controlling the potential of the control electrode. In the above image forming apparatus, it is preferable that the distance between the control electrode and the opposing electrode is kept uniform and constant. Further, keeping the position of the recording medium or the distance between the control electrode and the recording medium constant will provide excellent conditions for image forming.
However, the distance between the control electrode and the opposing electrode is so short that if plain paper or the like is used as the recording medium, the relative position of the recording medium to the control electrode or the opposing electrode will vary due to wrinkles, curls or elasticity of the recording medium itself and due to electric force exerting on the recording medium if it has charge. If the position of the recording medium changes, the size and shape of dots to be printed thereon varies, possibly causing failure to reproduce a desired image. Further, when the shape and size of dots to be formed vary, the reproduction performance of halftones lowers and make it impossible to create appropriate halftones, giving rise to difficulty in forming a satisfactory image.
If the position of the recording medium changes greatly, it may come in contact with the control electrode. In such a case, the toner which has already transferred to the recording medium is rubbed against the control electrode causing destruction of the image, and in addition, the toner adhering to the control electrode sticks to the recording medium causing background fogginess, or black lines or black smudges especially when the control electrode has been badly stained with toner.
Further, if the recording electrode and the control electrode come in contact with each other, the charge on the recording medium induces electricity on the control electrode surface, so that the charge on the control electrode surface will have potentials relative to the control electrode. As a result, the apparent potential of the control electrode relative to the toner supported on the toner support varies, deviating from the desired potential which has been applied to the control electrode. Accordingly, when the voltage for allowing toner to pass through the passage hole is applied, only an insufficient amount of toner may jump or the jumping itself might become difficult, causing image defect or printing failure, and hence resulting in difficulty in image forming. Even when the image is formed, the resultant image becomes blurred, lacking contrast, and faces difficulty in reproducing halftones because of insufficiency of toner transfer.
Further, when the apparatus is mounted under a high temperature, high humidity environment, the recording medium used absorbs moisture. This reduces its resistivity, and hence the control electrode and the opposing electrode start to conduct through the recording medium, so that the high voltage to the opposing electrode might leak to the control electrode, causing destruction of the devices such as the control electrode and control circuit and further destroying the other appliances connected to this apparatus. In the worst case, the user might be struck by electricity.
In the image forming apparatus disclosed in Japanese Patent Application Laid-Open Hei 5 No. 134,581, when a narrow-width recording medium such as postcards is used, the image forming region will have areas in which no recording medium exists during the printing operation. Since the recording medium must be conveyed by the rotation of the opposing electrode whilst being attracted to the opposing electrode, the recording medium needs to be attracted by a strong force. Therefore, air flow of more than a certain minimum should be formed around the region facing the control electrode. In this arrangement, in the ends of the image forming region where no recording medium exists as stated above, toner exists around the openings of the control electrode facing the aforementioned area while the toner on the developing sleeve is exposed through the opening. Therefore, if the air flow is directly formed with respect to the toner, not only the toner layer is stirred but the toner adhering to the openings and the toner residing on the developing sleeve in the areas facing the openings starts to jump by being sucked in by the air flow, and might transfer toward the opposing electrode thus possibly polluting it. Further, the toner might jump to the nearby recording medium, causing background fogginess and image degradation. This also causes unnecessary toner consumption.
In a conventional color image forming apparatus using a multiple number of developing vessels, there are cases where the developing operation and the cleaning operation are implemented simultaneously. For example, consider a case where development is effected using one of the color developing vessels whilst no recording medium exists on the opposing electrode surfaces facing the other colors of developing vessels. In this case, while the control electrode facing the opposing electrode being unused is being cleaned, the color toner could be scattered by the cleaning operation, for example, brushing. As a result, the scattered toner might transfer to the recording medium residing on the opposing electrode surface engaged in the printing, causing partial color fogginess as well as inducing color mixing.
In order to avoid the aforementioned problems, there is a configuration in which the opposing electrode is provided as a roller, and the recording paper is made to be attracted to this roller-shaped opposing electrode to thereby assure a stable, fixed position of the recording paper. That is, this opposing electrode is made up of a roller having mesh-like holes and a decompressing means for sucking the recording paper to the roller. This decompressing means suctions air through the mesh-like holes so that the recording medium will be attracted to the roller (opposing electrode). In this arrangement, the recording paper is conveyed by the rotation of the roller whilst being sucked.
This configuration, i.e., the roller-shaped opposing electrode is advantageous in conveying the recording medium, but the surface of the recording paper is set so as to be curved so that the distance between the control electrode and the recording paper will not be kept uniform. Therefore, the shape and density of dots formed at the central portion and at the edges become different, in particular, the dots at the edges lowers in density and becomes smaller in diameter, giving rise to difficulties in forming appropriate dots. This not only lowers the contrast and hence causes blurs in the image, but also degrades the reproduction performance of halftones. Furthermore, in the case of a color image forming apparatus, it becomes difficult to reproduce faithful colors.
In this case, this problem can be overcome by varying the voltage being applied to the control electrode so as to compensate for the distance between the control electrode and the recording paper. However, this method uses plural levels of voltages needing more power sources, and it is also necessary to enhance the voltage the driver to be used for switching the voltages can withstand, thus resulting in increased cost.
In order to avoid the above problems originating from the provision of a roller-shaped opposing electrode, there has been an attempt to use a static attraction conveying means in which a belt-shaped opposing electrode is provided while the recording electrode is electrified whereby the recording medium is adapted to be attracted electro-statistically to the belt for conveyance. This method, however, uses a belt and belt driving means, a charging means for recording medium, a charger power source and the like, needing more parts, causing sharp increase in cost. Further, it is impossible to attract the recording medium in some cases due to change in surrounding conditions and/or depending upon the material of the recording medium. In particular, the surface potential of the recording medium is liable to change depending upon its thickness and resistance, so this method involves potentially fatal problems such that the attraction itself becomes difficult.
As a method for avoiding the above various problems, an air suction conveying method has been proposed in which the recording medium is conveyed in a sliding manner whilst being suction applied onto an opposing electrode having a flat surface. In this method, however, since the recording paper is slid whilst being air-sucked, there is a problem that the recording paper vibrates due to frictional force. As the recording medium vibrates, it will separate from the suction port for air suction, markedly reducing the suction force exerted on the recording medium. Consequently, the recording medium will lift up with respect to the opposing electrode, varying the position of the recording medium, thereby causing similar image degradation to that stated above.